As a conventional pump for liquid chromatography has been known a construction in which a first plunger and a second plunger are driven independently and cooperatively by motors to reduce a pulsating flow rate (for example, see patent document 1).
Describing this construction, while the first plunger reciprocates one time, the second plunger also reciprocates one time to correct a pulsating flow rate caused by the suction of the first plunger by the operation of the second plunger. That is, in this construction, the first plunger determines a liquid flow rate and the second plunger is used for correcting the pulsating flow rate caused by the first plunger. [Patent document 1] Japanese Utility Model Laid-Open No. S63(1988)-36668 (pages 5 to 7)
However, in the above-described conventional pump for liquid chromatography, in a case where in order to feed liquid at a low flow rate, the speed reduction ratio of a motor is increased to decrease the speed of the plunger or the diameter or stroke of the plunger is decreased, conversely, the liquid can not be fed at a large flow rate. For this reason, this pump presents a problem that it takes much time to fill solvent into the passage of a measurement system on the downstream side of the pump at the startup of test and a problem that bubbles remaining in the pump can not be easily discharged. When the bubbles are not discharged, even if the plunger reciprocates, the plunger only compresses or expands the bubbles and cannot discharge the smallest amount of liquid. Therefore, there is presented a problem that this pump is not appropriate for constructing a pump used for an extremely low flow rate.
Moreover, a check valve for holding high pressure is generally made of a material of high hardness such as stainless steel, ruby, or ceramics. However, even if the check valve of this kind is closed, minute leakage cannot be avoided and in particular, in the case of feeding liquid at an extremely low flow, becomes a large factor to reduce a liquid feed accuracy. On the other hand, the check valve constructed of a soft material such as resin has high hermeticity and hence can minimize leakage but is not applicable because of durability. In this manner, there is presented a problem that the check valve cannot feed liquid at an extremely low flow rate with high accuracy.